The original 4x4 crossmembers bolted to both the top and the bottom flanges for a reason.

In my welding shop I've had the pleasure(?) of repairing the lower frame flange on 2 different Dodge 4x4 trucks where the owners (or someone) didn't think the bolts on the top flange were important enough to reinstall them after a trans rebuild. I don't believe the lower flange is strong enough to take the forces involved with the transfer case in action. Those two trucks may have been the strange ducks of the bunch, and who knows what extremes they may have seen, but I doubt Dodge would have invested the money to add that extra bracing if they didn't think it was needed. The extra cost of the extra bracket pieces and the extra bolds X the number of 4x4 trucks Dodge build was a big investment they would not have make if it wasn't deemed necessary.

Add the extra brackets. Especially since it looks like your crossmember bolts to the bottom side of the lower flange. Gene